Plasticized poly(vinyl butyral) sheets developed for glazings can generally fall into one of three categories based on the level of plasticizer contained in the composition, i.e., automobile, stiff, and acoustic poly(vinyl butyral). Automobile poly(vinyl butyral) sheets, such as those used as automobile windshields, are generally derived from poly(vinyl butyral) compositions containing greater than about 25 wt % of plasticizer. Stiff poly(vinyl butyral) sheets, such as those used as aircraft windshields, are generally derived from poly(vinyl butyral) compositions containing about 17 to about 23 wt % of plasticizer. Acoustic poly(vinyl butyral) sheets, such as those used as sound barrier layers in architectural and automobile glazings, are generally derived from poly(vinyl butyral) containing greater than about 45 wt % of plasticizer. The different amounts of plasticizer contained in these different types of poly(vinyl butyral) compositions provide the glazing interlayers derived therefrom with desirable penetration resistance at the temperature they are used. Thus, stiff poly(vinyl butyral) sheets, with a relatively low level of plasticizer, has optimum penetration resistance at high temperatures, while automobile poly(vinyl butyral) sheets, with a higher level of plasticizer, has optimum penetration resistance at lower temperatures.
Stiff poly(vinyl butyral) sheets have been disclosed in U.S. Pat. Nos. 4,130,684; 4,504,341; 4,960,631; and 5,270,518. In addition, Wong, et. al., in US-2005-0192398 A1, disclose a process to produce certain low color, stiff poly(vinyl butyral) sheets. The use of poly(vinyl butyral) compositions with low level of plasticizers has not been limited to glazing interlayers. For example, Degeilh, et. al., in U.S. Pat. No. 5,187,217, disclose a poly(vinyl butyral) composition with 16-20 wt % of plasticizer for gluing a base onto a glazing, such as a rearview mirror onto an automotive windshield.
Poly(vinyl butyral) compositions with low levels of plasticizer have also been disclosed for use in solar cell laminates. For example, Dran, et. al., in U.S. Pat. No. 4,321,418, disclose a process for producing a solar cell module, which involves the step of embedding interconnected solar cells between two (2) 5-mm-thick (197-mils-thick) layers of a mixture of 100 parts by weight of poly(vinyl butyral) resin powder and 20 parts by weight of powdered plasticizer made up of triethylene glycol di-(ethyl-2-butyrate). However, it is noted that the 5 mm thickness of the poly(vinyl butyral) resin composition could impair the efficiency of light transmission. Further, the process to produce such solar cell laminates is overly complicated for use in commercial settings.
As a renewable energy resource, the use of solar cell modules is rapidly expanding. With increasingly complex solar cell modules or laminates, also referred to as photovoltaic modules, comes an increased demand for enhanced functional encapsulant materials. Photovoltaic (solar) cell modules or laminates are units that convert light energy into electrical energy.
One typical or conventional construction of a solar cell laminate consists of voltage-generating component (or solar cell component) encapsulated by a polymer, such as poly(vinyl butyral) (PVB), with an incident layer (e.g., glass) and backing layer or back-sheet.
Plasticized poly(vinyl butyral) compositions have been disclosed as materials to form solar cell encapsulant layers. See, e.g., U.S. Pat. Nos. 3,957,537; 4,249,958; 4,321,418; 5,508,205; 5,582,653; 5,728,230; 6,075,202; 6,288,323; 6,288,326; 6,538,192; 6,777,610; 6,822,157; 6,940,008, US 2004-0191422, US 2005-0284516; EP 0 343 628; EP 0 631 328; EP 1 005 096; and EP 1 054 456. In these applications, the plasticized poly(vinyl butyral) compositions are mainly used as hot melt adhesives to encapsulate and protect the solar cells.
One preferred way of manufacturing a solar cell modules involves forming a pre-laminate comprising at least 5 structural layers. The solar cell pre-laminates are typically constructed in the following order starting from the top, or incident layer (that is, the layer first contacted by light) and continuing to the backing (the layer furthest removed from the incident layer): (1) incident layer (typically a glass plate or a thin polymeric film (such as a fluoropolymer or polyester film), but could conceivably be any material that is transparent to sunlight), (2) front-sheet (or first) encapsulant layer, (3) voltage-generating component (or solar cell component), (4) second encapsulant layer, and (5) backing layer or back-sheet.
The encapsulant layers of solar cell laminates are designed to encapsulate and protect the fragile voltage-generating component. Generally, a solar cell pre-laminate will incorporate at least two encapsulant layers sandwiched around the solar cell component. (Use of more than one such layer is also described in the art.) The optical properties of the front-sheet encapsulant layer must be such that light can be effectively transmitted to the solar cells. Over the years, a wide variety of polymeric films and sheets have been developed to produce laminated solar cell products. In general, these polymeric films and sheets must possess a combination of characteristics including very high optical clarity, low haze, high impact resistance, shock absorbance, excellent ultraviolet light resistance, good long term thermal stability, excellent adhesion to glass and other solar cell laminate layers, low ultraviolet light transmittance, low moisture absorption, high moisture resistance, excellent long term weatherability, among other requirements. Widely used encapsulant materials include complex, multi-component compositions based on ethylene vinyl acetate (EVA), acid copolymers and ionomers derived therefrom, poly(vinyl acetal) (preferably poly(vinyl butyral) (PVB)), polyurethane (PU), polyvinylchloride (PVC), polyethylene (e.g., metallocene-catalyzed linear low density polyethylene), polyolefin block elastomers, ethylene acrylate ester copolymers (e.g., poly(ethylene-co-methyl acrylate) and poly(ethylene-co-butyl acrylate)), silicone elastomers, epoxy resins, and the like.
Presently when pre-formed poly(vinyl butyral) sheets are used in preparing pre-laminates, highly plasticized poly(vinyl butyral) sheets are used to form the solar cell encapsulant layers. Due to the extreme softness and tackiness at ambient temperature, the use of pre-formed highly plasticized poly(vinyl butyral) sheets as solar cell encapsulant layers has been complicated by the need to refrigerate the highly plasticized poly(vinyl butyral) sheets during shipment and storage. In addition, highly plasticized poly(vinyl butyral) sheets, whose tackiness augments with rising temperature and whose sliding property towards glass gets worse, have a tendency to impair processability and workability. In order to improve the tendency and to suppress the hygroscopicity, the temperature of the working places must be maintained at about 20° C. In practice, when a highly plasticized poly(vinyl butyral) film or sheet is used as the intermediate layer between two sheets of glass, it is necessary to adopt a two-step bonding process to prepare the laminate (See, e.g., EP 0 145 928, page 2, line 30).
There is a need to develop products that overcome these shortcomings and to provide a simplified process for manufacturing solar cell modules.